The present invention relates to an aqueous zinc nitrite solution and to a method for preparing the same. More particularly, the present invention relates to an aqueous zinc nitrite solution containing only small amounts of sodium ions and sulfate ions as impurities and to a method for preparing the same. The present invention enables one to provide a very efficient surface treatment process by use of such an aqueous zinc nitrite solution.
Zinc nitrite is known to decompose at around 100xc2x0 C. with release of nitrogen oxide when slowly heated in air. The zinc nitrite is known to be soluble in water and vulnerable to hydrolysis and forms zinc oxynitrite [ZnO.Zn(NO2)2] upon evaporation of its aqueous solution.
Generally, such zinc nitrite is prepared by a method of mixing zinc sulfate with an ethanol solution of sodium nitrite, filtering the resulting precipitate, and evaporating and concentrating the filtrate to obtain crystals (cf. Kagaku Daijiten).
However, this method, which is a method in which sodium sulfate is separated and removed by utilizing the difference in solubility between zinc nitrite and sodium sulfate, requires the use of ethanol, and the operations of evaporation and concentration. The method has problems in that it inevitably causes an increase in cost and in that sodium ions are allowed to remain.
Therefore, it is currently difficult to obtain high purity zinc nitrite or its aqueous solution on an industrial scale.
Further, as a pretreatment process prior to painting of a metal surface, a series of steps of degreasing, washing with water, film forming treatment, washing with water, and drying is generally carried out. As an example of the film forming treatment, a treating method for forming a film of zinc phosphate on the surface of steel is commonly adopted. As a film-forming agent used for this purpose, a treating liquid prepared by dissolving zinc in phosphoric acid and diluting the resulting solution with water has been used. This treatment is called xe2x80x9cmetal surface treatmentxe2x80x9d.
Furthermore, in order to promote the film forming reaction of metal, a chemical such as sodium nitrite or sodium chlorate is added to the film-forming agent. These chemicals are called xe2x80x9cpromotersxe2x80x9d.
However, conventionally used sodium salts such as sodium nitrite and sodium chlorate have problems in that long usage of a treating bath increases the concentration of Na ions and as a result the pH of the treating bath is elevated so that components of the formed film precipitate in the treating bath. When recovering and regenerating the old treating liquid, accumulation of Na ions in the treating bath destroys the balance of the bath so that removal of Na ions from the recovered treating liquid is necessary. Usually, treating liquid containing Na ions has to be disposed of as industrial waste.
Recently, the issue of environmental protection has attracted much attention, even in the field of metal surface treating liquids and attempts have been made to establish a closed system for treating baths.
In view of the above, the present inventors have intensivly researched aqueous zinc nitrite solutions containing substantially no sodium ions and as a result they have found an aqueous zinc nitrite solution containing only a small amount of sodium ions or sulfate ions and a method for preparing the same.
They also have found that when the solution is used as a promoter for metal surface treatment, a very efficient surface treatment process can be provided. The present invention has been completed based on these findings.
That is, the present invention relates to an aqueous zinc nitrite solution characterized in that the solution contains as impurities 10 ppm or less of sodium (Na) and 100 ppm or less of sulfate ions (SO42xe2x88x92) calculated when the aqueous solution has a concentration of zinc nitrite [Zn(NO2)2] set out 10% by weight in terms of NO2.
The present invention also relates to a method for preparing an aqueous zinc nitrite solution, comprising a first step of reacting zinc sulfate with calcium nitrite to form an aqueous zinc nitrite solution and a second step of purifying the aqueous zinc nitrite solution.
In the present invention, barium ions may be preliminarily added in the first step of reacting zinc sulfate with calcium nitrite in an amount 1.05 times or more of the equivalent amount of sulfate ions dissolved in the reaction mixture after the reaction to form an aqueous zinc nitrite solution.
In the present invention, the reaction may be carried out at a Ca/Zn ratio in a starting material within the range of 0.5 to 1.5 in the first step of reacting zinc sulfate with calcium nitrite to form an aqueous zinc nitrite solution.